Alkylation process for production of motor fuels

ABSTRACT

A liquid phase alkylation process for production of alkylate useful as motor fuel, wherein low molecular weight isoparaffin hydrocarbons are alkylated with olefin hydrocarbons in the presence of a catalyst, comprising a mixture of a major portion of sulfuric acid (H 2  SO 4 ) with a minor portion of trifluoromethane sulfonic acid (CF 3  SO 3  H) which may contain about 0-3 weight percent water and about 0-10 weight percent acid oils.

This is a division, of application Ser. No. 561,401, filed Mar. 24, 1975now U.S. Pat. No. 3,970,721.

This application is related to U.S. patent application, Ser. No.520,595, now U.S. Pat. No. 3,922,319 filed Nov. 4, 1974.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to liquid phase, acid catalyzed alkylationof isoparaffin hydrocarbon with olefin hydrocarbon for production ofalkylate having high octane value and being suitable for gasolineblending stock. More particularly, the present invention relates to suchan alkylation process wherein improved acid catalyst comprising aselected ratio of trifluoromethane sulfonic acid with sulfuric acid isemployed.

2. DESCRIPTION OF THE PRIOR ART

Liquid phase alkylation processes wherein isoparaffin hydrocarbons, suchas isobutane, etc. are alkylated with olefin hydrocarbons such aspropylene, butylenes, etc. for production of alkylate productscomprising highly branched C₇ -C₈ range paraffin hydrocarbons havinghigh octane values, are well known and widely practiced. In suchalkylation processes, reactant hydrocarbons are generally contacted inthe liquid phase, at temperatures in the range of from about 0°-100° F.,in the presence of acid alkylation catalysts under conditions of goodmixing to produce the desired alkylate hydrocarbons. Reaction pressures,which may vary from about ambient to superatmospheric, are sufficient tomaintain reactants in the liquid phase. Higher pressures than thatrequired to maintain reactants in the liquid phase apparently have nosubstantial effect upon such alkylation reactions. Acid catalysts may beselected from a wide range of strong acids including sulfuric acid andfluorosulfonic acid. Generally, liquid alkylation catalysts such assulfuric acid, trifluoromethane sulfonic acid and fluorosulfonic acidare substantially immiscible with isoparaffin hydrocarbon reactants.Consequently, reactant hydrocarbons and liquid alkylation catalysts arecontacted in an alkylation zone under conditions of high shear mixingsufficient to form an emulsion of immiscible hydrocarbon and catalyst.Detailed description of such alkylation processes is not requiredherein, as such processes are well known and widely practiced in theprior art.

In such alkylation processes side reactions take place in addition todesired alkylation reactions wherein 1:1 olefin-isoparaffin adducts areproduced. For example, olefin hydrocarbons tend to polymerize in thepresence of strong acid catalysts to form C₁₂ and higher molecularweight polymers; also such polymers tend to crack in the presence of thestrong acid catalyst forming low octane C₅ -C₇ hydrocarbons. These sidereaction products, e.g., olefin polymers and cracked products, haverelatively low octane values and are not as desirable as the 1:1 olefinisoparaffin alkylation products for use as gasoline blending stocks.

In order to maximize the yield of the desired alkylate products at theexpense of the undesired side reaction products, care is taken to insuregood contact of isoparaffin reactants with olefin reactants in thepresence of alkylation catalysts under reaction conditions which favorformation of alkylate hydrocarbon products over side reaction products.For example, reaction temperatures are maintained in the range of about0° F. to about 100° F. wherein the alkylation of isoparaffins witholefins proceeds at economically attractive rates and wherein olefinpolymerization rates are not so great as to consume excessive amounts ofolefin reactant. Further, a substantially stoichiometric excess ofisoparaffins over that required to react with olefin is employed toinsure contact of isoparaffins with olefins under alkylation reactionconditions. Further, as olefin reactants are generally substantiallymore soluble in acid catalysts than are isoparaffin reactants,high-shear mixing of the alkylation reaction mixture is generallyprovided sufficient to form emulsions of acid catalysts and hydrocarbonreactants. Formation of such emulsions increases the contact areabetween acid catalyst, containing dissolved olefins with isoparaffinhydrocarbons, thus materially improving yield and quality of alkylateproduct.

Commonly, where sulfuric acid is the selected acid catalyst, thealkylation reaction emulsion is one wherein isoparaffin hydrocarbon isemulsified in a continuous acid phase, although it is known to employreaction emulsions comprising acid catalyst phase suspended in acontinuous isoparaffin hydrocarbon phase. Alkylation reaction mixturescomprising about 40-70 volume percent sulfuric acid catalyst and about60-30 volume percent hydrocarbon phase are known to form emulsionswherein the acid phase is continuous.

It is well-known that sulfuric acid is a particularly effective catalystfor the alkylation of isoparaffin hydrocarbon with olefin hydrocarbon.Additionally, it is known that sulfonic acids, including fluorosulfonic(HFSO₃), trifluoromethane sulfonic (CF₃ SO₃ H) chlorosulfonic, andvarious organic sulfonic acids are effective alkylation catalysts usedalone or in admixture with other strong acids. For example see: U.S.Pat. No. 3,708,533, Olah, which teaches alkylation of paraffins witholefins in a reaction catalyzed by a mixture of Lewis acid (e.g. SbF₅)and a Bronsted acid (e.g. CF₃ SO₃ H); U.S. Pat. No. 2,313,013 whichteaches alkylation of an isoparaffin with an olefin hydrocarbonemploying fluorosulfonic acid as catalyst is superior to alkylationreactions employing H₂ SO₄ catalyst; U.S. Pat. No. 3,231,633 wherein itis taught that the catalytic activity of fluorosulfonic acid is improvedby addition of a minor amount of H₂ SO₄ (e.g. not less than 50 weightpercent HFSO₃ is the catalyst mixture); U.S. Pat. No. 2,259,723, Ballardet al, which teaches the use of chlorosulfonic and fluorosulfonic acidsin admixture with hydrogen halides; U.S. Pat. No. 3,766,293, Parker etal which teaches the use of "fluorosulfuric" acid in combination with aminor amount of catalyst promoter which may include sulfuric acid; U.S.Pat. No. 3,231,633 Kramer, which teaches an alkylation process employingfluorosulfonic acid in combination with up to about 50 weight percentsulfuric acid; and U.S. Pat. No. 2,425,572, Slotterbeck, which teachesan alkylation process wherein a minor portion (1-10%) of chlorosulfonicacid or other substituted sulfonic acids, such as aliphatic sulfonicacids, are added to the sulfuric acid alkylation catalyst.

Trifluoromethane sulfonic acid, as well as other sulfonic acids, isrelatively expensive compared to sulfuric acid. Thus, one of the majordisadvantages of using trifluoromethane sulfonic acid, or other sulfonicacids, as alkylation catalyst is high cost of these acids. In alkylationreactions, acid catalyst is consumed at rates ranging from about 0.1 upto about 1.0 or more pounds of catalyst per gallon of alkylate productas a result of side reactions such as hydrolysis and reactions witholefin hydrocarbons. Consequently, the expense of providing sulfonicacids as the major components of alkylation catalysts is generallyprohibitively expensive in commercial alkylation processes.

SUMMARY OF THE INVENTION

Now, according to the present invention, we have discovered an improvedalkylation process wherein C₄ -C₆ isoparaffin hydrocarbons are alkylatedwith C₃ -C₅ olefin hydrocarbons in the presence of a novel alkylationcatalyst comprising a major portion sulfuric acid and a minor portiontrifluoromethane sulfonic acid, and which may contain about 0-3 weightpercent water and about 0-10 weight percent acid oils.

In a preferred embodiment, an isoparaffin hydrocarbon such as isobutaneis contacted with olefin hydrocarbons such as propylene, butylenes, andmixtures thereof in a molar ratio of isoparaffin to olefin of from about2:1 to 20:1 in the liquid phase, in the presence of an alkylationcatalyst comprising trifluoromethane sulfonic acid and H₂ SO₄ in aweight ratio of 0.166 to 1 at a temperature in the range from about 0°F. to about 100° F. for production of alkylate hydrocarbon producthaving high octane valve and being suitable for use in gasoline motorfuel. Preferably, the alkylation reaction mixture is subjected to goodmixing to form a hydrocarbon in continuous acid phase emulsion whichcomprises from about 40 to about 70 volume percent acid phase and fromabout 60-30 percent hydrocarbon phase. Residence time of reactanthydrocarbons in the presence of acid catalyst under alkylation reactionconditions is preferably such that olefin space velocity is in the rangeof about 0.1 to about 1.0 volumes olefin per hour per volume ofcatalyst.

Advantages of the present invention include alkylate products ofincreased octane values compared to alkylation reactions employing H₂SO₄ catalyst alone. Additionally, by employing the improved catalyst ofthe present invention, corrosion of steel and stainless steel issubstantially reduced, compared to acid catalysts comprising mixtures ofHFSO₃ and H₂ SO₄. These, and other advantages will be discussed in thedetailed description which follows.

DETAILED DESCRIPTION OF THE INVENTION

The improvement of the present invention comprises use of a novelalkylation catalyst in processes for alkylating C₄ -C₆ isoparaffinhydrocarbons with C₃ -C₅ olefin hydrocarbons for production of highoctane alkylate products suitable for use in blending of gasoline motorfuel.

Now, according to the present invention, we have discovered analkylation catalyst comprising sulfuric acid and trifluoromethanesulfonic acid which is effective for producing alkylate of increasedoctane value, and which is only as corrosive toward carbon steel as isH₂ SO₄. The novel catalyst of the present invention comprises a majorportion sulfuric acid in admixture with a minor portion trifluoromethanesulfonic acid preferably in a weight ratio of trifluoromethane sulfonicacid to sulfuric acid of 0.166:1. Such catalyst may contain from about 0to about 3 weight percent water and from about 0 to about 10 weightpercent acid oils. The acid oils are relatively high molecular weightreaction products of sulfuric acid with hydrocarbons present in thealkylation process.

In addition to improved alkylate quality over alkylation reactionsemploying sulfuric acid alone as catalyst, the catalyst mixture of thepresent invention is substantially less corrosive to steel and stainlesssteel equipment than are the H₂ SO₄ --HFSO₃ catalyst mixtures disclosedin U.S. Patent Application, Serial No. 520,595, now U.S. Patent No.3,922,319 filed Nov. 4, 1974. The acid catalyst of this invention maycontain from about 1 to about 3% water without being seriouslydeactivated. As the catalyst of the present invention may containsubstantial amounts of water without seriously affecting catalyticactivity, such fresh catalyst may be conveniently formulated withcommercially available sulfuric acid of about 99.5-98 weight percentpurity.

Trifluoromethane sulfonic acid, having the chemical formula CF₃ SO₃ H,which may be used in the practice of the present invention need not becompletely pure. Commercial grades of trifluoromethane sulfonic acidhaving purities of about 99 weight percent may be used to formulate thecatalyst of the present invention.

Sulfuric acid which is useful in formulating the fresh catalyst of thepresent invention is concentrated sulfuric acid, preferably of about99.5-98 weight percent H₂ SO₄ and containing about 0.5-2 weight percentwater.

Alkylation processes contemplated in the present invention are thoseliquid phase processes wherein isoparaffin hydrocarbons such asisobutane, isopentane, isohexane, etc. are alkylated with mono-olefinshydrocarbons such as propylene, butylenes, pentylenes, etc. forproduction of high octane alkylate hydrocarbons boiling in the gasolinerange and which are suitable for use in gasoline motor fuel. Preferably,isobutane is selected as the isoparaffin reactant and olefin reactant isselected from propylene, butylenes, and mixtures thereof for productionof an alkylate hydrocarbon product comprising a major portion of highlybranched aliphatic C₈ hydrocarbons having high octane value. Such olefinreactants in the presence of strong acids and particularly sulfuricacid, tend to undergo undesirable alkylation reactions with isoparaffinhydrocarbons. In order to increase selectivity of olefin hydrocarbonconversion for alkylation with isoparaffin hydrocarbon, a substantialstoichiometric excess of isoparaffin hydrocarbon is desirable in thereaction zone. Liquid volume ratios of isoparaffin hydrocarbon to olefinhydrocarbon of from about 2:1 about 20:1 are contemplated in the presentinvention. Isoparaffin and olefin reactant hydrocarbons normallyemployed in commercial alkylation processes are derived from refineryprocess streams and usually contain small amounts of impurities such asnormal butane, propane, etc. Such impurities are undesirable in largeconcentration as they dilute reactants in the reaction zone, thusdecreasing reactor capacity available for the desired reactants andinterferring with good contact of isoparaffin with olefin reactants.Additionally, in continuous alkylation processes wherein excessisoparaffin hydrocarbon is recovered from an alkylation reactioneffluent and recycled for contact with additional olefin hydrocarbon,such nonreactive normal paraffin impurities tend to accumulate in thealkylation system. Consequently, process charge streams and/or recyclestreams which contain substantial amounts of normal paraffin impuritiesare usually fractionated to remove such impurities and maintain theirconcentration at a low level, preferably less than about 5%, in thealkylation process.

Alkylation reaction temperatures within the contemplation of the presentinvention are in the range of from about 0° F. to about 100° F. Lowertemperatures favor alkylation reaction of isoparaffin with olefin overcompeting olefin side reactions such as polymerization. However, overallreaction rates decreases with decreasing temperatures. Temperatureswithin the given range, and preferably in the range from about 40° F. toabout 70° F., provide good selectivity for alkylation of isoparaffinwith olefin at commercially attractive reaction rates.

Reaction pressures contemplated in the present invention are sufficientto maintain reactants in the liquid phase and may range from ambient tosuperatmospheric. Reactant hyrocarbons may be normally gaseous atalkylation reaction temperatures, thus reaction pressures in the rangeof about 10-150 psig are preferred. With all reactants in the liquidphase, increased pressure has no significant effect upon the alkylationreaction.

Contact times for hydrocarbon reactants in an alkylation zone, in thepresence of the alkylation catalyst of the present invention may rangefrom about 0.5 to about 60 minutes. Preferably, contact time issufficient for essentially complete conversion of olefin reactant in thealkylation zone. In the alkylation process of the present invention,employing isoparaffin and olefin reactants in volume ratios in the rangeof 2:1 to 20:1, wherein the alkylation reaction mixture comprises about40-70 volume percent acid catalyst phase and about 60-30 volume percenthydrocarbon phase, and wherein good contact of olefin with isoparaffinis maintained in the reaction zone, essentially complete conversion ofolefin may be obtained at olefin space velocities in the range of about0.1 to 1 volume olefin per hour per volume catalyst (v/v/hr.). Optimumspace velocities will depend upon isoparaffin and olefin reactants, theparticular compositions of alkylation catalyst, and alkylation reactionconditions. Consequently, the preferred contact times are sufficient forproviding an olefin space velocity in the range of 0.1 to 1 (v/v/hr) andallowing essentially complete conversion of olefin reactant in thealkylation zone.

Batch or continuous methods of operation may be used. Differentprocedures, which may vary with the method of operation, may be used forensuring the presence of acid catalyst having the desired acidity andratio of H₂ SO₄ to CF₃ SO₃ H in the reaction mixture. Thus, in batchoperations, acid catalyst having the desired ratio of H₂ SO₄ to CF₃ SO₃H may be added with isoparaffin to a reaction vessel. Olefin reactantscan then be added at a desired rate to the reaction mixture for a timeuntil acidity of the acid catalyst declines, or until a substantialportion of the isoparaffin reactant is consumed.

In a continuous process, isoparaffin and acid catalyst are continuouslycharged separately to an alkylation zone maintained at alkylationreaction conditions. Olefin hydrocarbon may be separated charged, or maybe added in admixture with the isoparaffin at a rate to maintain thedesired olefin space velocity in the alkylation zone. Reaction effluentis continuously removed from the alkylation zone and separated into anacid phase and a hydrocarbon phase comprising unreacted isoparaffin andalkylated hydrocarbon. A portion of the separated acid phase is recycledfor contact with additional hydrocarbon reactants. Preferably, therecycled acid phase contains from 5 to 30 volume percent hydrocarbon forimproving contact of isoparaffin with olefin in the presence of the acidcatalyst. A portion of the acid phase may be continuously orintermittently withdrawn from the process for removal of water and acidoil impurities. Fresh acid catalyst, comprising H₂ SO₄ and CF₃ SO₃ H inthe selected ratio and having high acidity is added to make up for theacid phase withdrawn, thereby maintaining the acidity of the acidcatalyst in the alkylation zone within the desired range of acidity. Theseparated hydrocarbon phase may be fractionated in a fractionation zoneinto an alkylate fraction and an unreacted isoparaffin fraction.Additionally, hydrocarbon impurities, such as normal paraffins, may befractionated from isoparaffins in the fractionation zone. Preferably,fractional distillation methods are employed to fractionate theseparated hydrocarbon phase. The isoparaffin fraction may be recycled tothe alkylation zone for contact with additional reactants.

The desired agitation of the reaction mixture, for insuring good contactof isoparaffin with olefin in the presence of alkylation catalyst, maybe provided in a number of ways. For instance, mechanical mixers may beused for forming a reaction emulsion of hydrocarbon and acid catalyst inthe alkylation zone. Additionally, charge and recycle streams may becharged through nozzles or jets into the reaction zone for providing thedesired agitation. These, and other methods for obtaining goodliquid-liquid mixing in the alkylation zone may be used advantageouslyin the method of the present invention.

The following examples demonstrate the advantages of the presentinvention. These examples are by way of illustration only, and are notintended as limitations upon the invention as set out in the appendedclaims.

EXAMPLE I

A series of continuous alkylation runs was performed in the liquidphase, at about 50° F., employing as reactant charge stocks a petroleumrefinery butylene stream, C.P. grade propylene and technical gradeisobutane. The purpose of these runs was to compare alkylation reactionsemploying sulfuric acid catalyst, H₂ SO₄ --HFSO₃ mixed acid catalyst andH₂ SO₄ --CF₃ SO₃ H mixed acid catalysts of the present invention. In allthese alkylation runs, acid catalyst and reactant hydrocarbons werecontacted in a carbon steel alkylation reactor with mechanical mixing toform a reaction emulsion. Reaction emulsion was continuously withdrawnand separated by gravity settling, into a catalyst phase and ahydrocarbon phase. Separated catalyst phase was recycled to thealkylation reaction for contact with additional reactant hydrocarbons.

For alkylation runs employing H₂ SO₄ catalyst, fresh catalyst makeup was97.9% H₂ SO₄ containing about 2.1% water. For alkylation runs employingH₂ SO₄ --HFSO₄ mixed acid catalyst, fresh catalyst comprised 14.3 weightpercent HFSO₃, 83.9 weight percent H₂ SO₄, and 1.8 weight percent H₂ O,which results in a fresh catalyst having 0.172:1 weight ratio offluorosulfonic acid to sulfuric acid and titratable acidity of 21.53meg/gm. For alkylation runs employing the H₂ SO₄ --CF₃ SO₃ H mixedcatalyst of the present invention, fresh catalyst comprised 14.0 wt. %,CF₃ SO₃ H, 84.2 wt. % H₂ SO₄, and 1.8 wt. % H₂ O which results in afresh catalyst having 0.166/1 weight ratio of CF₃ SO₃ H to H₂ SO₄ and acalculated titratable acidity of 18.1 meg/gm.

Operating conditions and results of this first series of alkylationruns, are set-out in Table 1 below. Butylene space velocities (volumeolefin per hour per volume catalyst) were maintained relativelyconstant, as were reaction temperatures and isobutane-to-butylene volumeratios.

                                      TABLE 1                                     __________________________________________________________________________                      Olefin                                                                        Space          Alkylate                                     Run                                                                              Temp.                                                                             Catalyst   velocity                                                                           Isoparaffin/olefin                                                                      Octane                                       No.                                                                              ° F.                                                                       Type  Olefin                                                                             v/v/hr                                                                             weight ratio                                                                            RON-CLEAR                                    __________________________________________________________________________    1  50°                                                                         CF.sub.3 SO.sub.3 H--                                                              Butylene                                                                           0.53 5.12/1    96.6                                                --H.sub.2 SO.sub.4                                                     2  50°                                                                         CF.sub.3 SO.sub.3 H                                                                Butylene                                                                           0.21 6.82/1    89-93.7                                      3  50°                                                                         H.sub.2 SO.sub.4                                                                   Butylene                                                                           0.43 5.12/1    95.8                                         4  50°                                                                         HFSO.sub.3 --                                                                      Butylene                                                                           0.52 5.12/1    96.7                                                --H.sub.2 SO.sub.4                                                     __________________________________________________________________________

From Table 1, comparison of Run 1 with Runs 2 and 3, all made withbutylene as olefin charge stock, demonstrates the CF₃ SO₃ H--H₂ SO₄mixed acid of the present invention produces alkylate of higher octanevalue than the alkylate product obtained from alkylation reactionsemploying either CF₃ SO₃ H or H₂ SO₄ alone as catalyst. Comparison ofRun 1 with Run 4 demonstrates the catalytic activity of the CF₃ SO₃H--H₂ SO₄ mixture of the present invention is about equivalent tocatalytic activity of a HFSO₃ --H₂ SO₄ mixed catalyst.

Thus, the mixed CF₃ SO₃ H--H₂ SO₄ catalyst produces higher octanealkylate than conventional H₂ SO₄ catalyst.

EXAMPLE II

Spent acid catalysts from Runs 1, 3 and 4 of Example I, table 1 each ofwhich contained stainless steel corrosion coupons were recovered free ofentrained hydrocarbon and were analyzed by atomic absorptionspectroscopy for iron, chromium, and nickel. The relative amounts ofthese metals for each run is proportional to the severity of corrosionof the carbon steel reactor and the stainless steel corrosion coupons bythe respective acid catalyst system. Data from these analyses are shownin Table 2.

                  TABLE 2                                                         ______________________________________                                                       Spent Catalyst Metals                                          Alkylation                                                                            Composition  Iron     Nickel Chromium                                 Run     Spent Catalyst                                                                             wt. ppm  wt. ppm                                                                              wt. ppm                                  ______________________________________                                        1       CF.sub.3 SO.sub.3 H--H.sub.2 SO.sub.4                                                      169      <10    68                                       3       H.sub.2 SO.sub.4                                                                           144      <10    29                                       4       HFSO.sub.3 --H.sub.2 SO.sub.4                                                              6700      165   365                                      ______________________________________                                    

From Table 2, it is seen that the amount of metals in the CF₃ SO₃ H--H₂SO₄ catalyst of the present invention is very close to the amount ofmetals in the H₂ SO₄ catalyst, and is much less than the amount ofmetals found in the HFSO₃ --H₂ SO₄ catalyst. The CF₃ SO₃ H--H₂ SO₄catalyst is substantially less corrosive than the HFSO₃ --H₂ SO₄catalyst, and is only about as corrosive as H₂ SO₄ catalyst. It is wellknown in the petroleum refining industry that corrosion rates in therange shown for H₂ SO₄ and CF₃ SO₃ H--H₂ SO₄ catalysts, are tolerable incommercial alkylation processes, and that carbon steel vessels, piping,and equipment may be used.

Thus, it is seen in Tables 1 and 2 that alkylate products of improvedoctane value over the H₂ SO₄ catalyst may be obtained using the CF₃ SO₃H--H₂ SO₄ catalyst of the present invention, without the drawback ofincreased corrosion caused by the HFSO₃ --H₂ SO₄ catalyst. Additionally,by employing the catalyst of the present invention, comprising CF₃ SO₃ Hand H₂ SO₄ in a weight ratio of about 0.166/1 the cost of the catalystmay be substantially reduced over costs of catalysts comprising a majorportion of halosulfonic and/or substituted halo-sulfonic acids,

It will be apparent to those skilled in the art that many variations andmodifications of the present invention, which has a principle featurethe use of an acid catalyst comprising a minor portion CF₃ SO₃ H and amajor portion of H₂ SO₄, preferably in a weight ratio of about 0.166/1respectively in a process for alkylating isoparaffin hydrocarbon witholefin-hydrocarbons.

We claim:
 1. An alkylation catalyst consisting essentially of a mixtureof CF₃ SO₃ H and H₂ SO₄, in a weight ratio of about 0.166/1respectively, containing about 0-3 weight percent water and about 0-10weight percent acid oils.